Many grinding operations are conducted in rotating/tumbling mills, generally in the form of hollow cylindrical drums. In a grinding mill operation, ore is introduced at a feed or charge end and comminuted (reduced in size) ore exits the mill at a discharge end. Typically, the mill is rotated about a horizontal axis or at a slight angle to the horizontal. During rotation, the material within the mill is lifted or carried up the ascending side of the shell and is tumbled back down to impact the “toe” of the charge (bottom of the mill). The fall of ore and/or its collision with grinding aids, if used, crushes the ore to a smaller size. This comminution process can involve various mechanisms including abrasion, attrition, impact forces, and so forth.
Ore grinding can be classified as autogenous grinding (AG), semi-autogenous (SAG), or media enhanced. AG processes do not use grinding aids (media). Rather, the material (ore) itself performs the crushing. SAG mills employ relatively small amounts of grinding media, such as, for instance, steel balls of five inches in diameter. A typical SAG mill uses a grinding media charge of about 8 to 20 percent (%) of the total volume of the mill. In contrast, media enhanced grinding uses a large percentage (typically about 30%) of smaller grinding aids, such as pebbles, balls (ball mills) or rods (rod mills).
In mining applications, grinding mills can also be classified as primary or secondary types, with primary mills receiving heavy mineral, typically ten inches in size or more and producing crushed material of, for example, about ¼ inch in size. Secondary mills receive the smaller ore material and further grind it, e.g., to a powder. Minerals may then be extracted from the powder by chemical or flotation techniques or by other methods, as known in the art. AG and SAG mills are commonly used in the primary stage, while ball or rod mills are used in the secondary stage.
The interior of a grinding mill such as, for instance, an AG or SAG mill, is provided with steel or rubber liners disposed longitudinally (parallel to the cylinder axis) and fitted with lifter bars. Due to the large size and weight of the liners, they are often formed from liner components (elements or segments) that can be individually secured to the drum or shell of the mill, e.g., by bolts or other suitable attachments. Liners and/or elements thereof (e.g., lifter bars) perform various functions, including those of protecting and insulating the shell, sealing the shell against corrosive charge, lifting and releasing portions of the charge, and so forth. Typical arrangements of liners and liner components are described, for example, in U.S. Pat. No. 5,832,583 issued on Nov. 10, 1998 to Wason and U.S. Application Publication No. 2012/0228416 A1 to Page et al., published on Sep. 13, 2012, both documents being incorporated herein by reference in their entirety.
Traditionally, when rubber (elastomeric) liner elements such as lifters (also referred to herein as lifter bars) are utilized, the type of rubber employed is a blend of natural rubber (NR) and butadiene rubber (BR), reinforced with carbon black (CB). Generally, blends of NR-BR can contain the two polymers in various ratios, depending on the application. In a typical liner element such as a lifter bar, the ratio BR:NR is about 40:60.
Over time, exposure to the abrasion and impact forces generated during grinding leads to the wear of the liners and/or elements thereof, as described, for instance, in U.S. Pat. No. 5,832,583. Wear patterns were investigated by M. Yahyaei and S. Banisi in the article Spreadsheet-Based Modeling of Liner Wear Impact on Charge Motion in Tumbling Mills, Mineral Engineering Vol. 23, pp. 1213-1219 (2010). The results presented in the latter document indicated, for instance, that after 4,000 hours of operation, the lifter face angle could increase from 14° to 47.1° and the height of lifters decrease from 15.2 to 5.8 cm. In addition, the article reported a non-uniform wear profile along the length of the mill.
Eventually a point is reached when worn liners or elements thereof must be replaced. In primary grinding, for example, lifters are usually removed from the mill when the lifting effect of the lifting bars is reduced through wear to approximately two to two and one-half inches in height. In many instances, the useful life of a lifter bar operated under aggressive grinding conditions is about 6 months. With less severe grinding, lifter bars can last up to about 2 years before needing to be replaced.
A variety of configurations of grinding mills and mill components such as liners, wear elements, and lifter bars, have been developed with the goal of improving performance, including increasing performance lifetimes. Examples include US patents, US published applications and PCT Publications Nos. WO2006/076764, U.S. Pat. No. 8,016,220, WO2006/076763, U.S. Pat. No. 7,997,517, WO2009/050723, US2010233420, WO2009/008810, U.S. Pat. No. 8,152,086, WO2007/048874, U.S. Pat. No. 7,887,000, their teachings being incorporated herein by reference.
Nevertheless, the mill needs to be stopped while the linings are replaced. This is time consuming and labor intensive, often requiring special equipment. It is associated with significant costs and impacts the overall productivity of the mill, especially if downtime for replacing wear components is unscheduled.